Showing papers on "Isomerization published in 1980"

Cis-trans isomerism in (trpy)(PPh3)RuC12. Comparisons between the chemical and physical properties of a cis-trans isomeric pair

Abstract: The syntheses of complexes derived from the planar, tridentate ligand 2,2',2''-terpyridine (trpy) of the type (trpy)Ru/sup II/(L)Cl/sub 2/ where L = PPh/sub 3/, P(p-C/sub 6/H/sub 4/Me)/sub 3/, or pyridine are described. Some rather striking differences in the properties of the cis and trans-isomers of the phosphine derivatives are reported. (BLM).

Homogeneous Catalysis: The Applications and Chemistry of Catalysis by Soluble Transition Metal Complexes

Abstract: Trends in Homogeneous Catalysis in Industry. Isomerization of Olefins. Reactions of Olefins and Dienes--Hydrogenation and HY Additions. Polymerization and Oligomerization of Olefins and Dienes. Reactions of Carbon Monoxide. Oxidation of Olefins and Dienes. Arene Reactions. Reactions of Acetylenes. Carbene Complexes in Olefin Metathesis and Ring-Forming Reactions. Oxidation of Hydrocarbons by Oxygen. Esterification, Polycondensation, and Related Processes. Homogeneous Catalysis in Halocarbon Chemistry. Appendix. Index.

Folding mechanism of the triple helix in type-III collagen and type-III pN-collagen. Role of disulfide bridges and peptide bond isomerization.

Abstract: The kinetics of triple-helix formation in type III pN-collagen, type III collagen and a quarter fragment of type III collagen was followed by optical rotation and circular dichroism. Kinetic intermediates were detected by trypsin digestion and polyacrylamide gel electrophoresis. The end products of refolding at 25 degrees C were identical to the native molecules according to their melting profiles, molecular weights and sedimentation behavior. Only at low temperatures (4-15 degrees C) were mismatched structures of lower stability formed. At 25 degrees C helix formation started exclusively at the set of three disulfide bridges which link the three chains at the carboxy-terminal end. The growth of the triple helix proceeds from this single nucleus at a rather uniform rate in a zipper-like fashion. This gives rise to zero-order kinetics over a large fraction of the conversion. Consequently the time of half conversion is proportional to the length of the molecule. From the appearance and disappearance of intermediates the growth of the triple helix could be observed directly. The rate of helix propagation is determined by the rate of cis leads to trans isomerization of peptide bonds. A model mechanism was devloped which quantitatively described the overall kinetics as well as the time course of the intermediates with a single set of parameters: the rate constant of cis leads to trans isomerization k = 0.015 s-1 and an average number of 30 tripeptide units in uninterrupted stretches of residues with all peptide bonds in trans configuration.

Resonance Raman evidence for an all-trans to 13-cis isomerization in the proton-pumping cycle of bacteriorhodopsin.

Abstract: Using a dual-beam flow technique, we have obtained resonance Raman spectra of the M412 photointermediates of both native purple membrane (15H M412) and purple membrane regenerated with 15-deuterioretinal (15D M412). For comparison, we have also obtained Raman spectra of the n-butylamine Schiff bases of the 13-cis and all-trans isomers of 15H and 15D retinal. The 15D model compound spectra, when compared to the 15H spectra, show isotopically induced spectral changes that are markedly different for the two isomers. There is a very close agreement between the frequency and intensity changes which occur upon deuteration of M412 and those which occur upon deuteration of the 13-cis model compound, but not even a qualitative correspondence exists when M412 and the all-trans model compound are similarly compared. These data demonstrate that the chromophore of M412 is an unprotonated Schiff base of 13-cis-retinal rather than all-trans-retinal. An analogous spectral comparison of 15H and 15D light-adapted bacteriorhodopsin (bRLA) with the 15H and 15D protonated Schiff bases of 13-cis- and all-trans-retinal demonstrates that bRLA contains an all-trans chromophore, in agreement with previous extraction experiments. Thus, a trans leads to cis isomerization occurs in the proton-pumping photocycle of Halobacterium halobium.

Oligomerized olefins for lubricant stock

Abstract: A process is disclosed for the production of high quality synthetic lubricants which comprises transforming ethylene into a mix of alpha olefins, separating from this mix of alpha olefins the alpha olefins in the range from about 14 to 20 carbon atoms, oligomerizing the alpha olefins in the range from about 14 to 20 atoms with a perfluorosulfonic acid resin catalyst and hydrogenating these oligomerized olefins. The olefins from the original mix of alpha olefins which are lighter and heavier than the 14 to 20 carbon atom alpha olefins processed above are combined and subjected to an isomerization/disproportionation process and the resulting olefins in the range of about 14 to 20 carbon atoms are oligomerized with perfluorosulfonic acid resin catalyst as above. Optionally, these olefins in the range of 14 to 20 carbon atoms may be combined with the olefins prepared directly from ethylene before oligomerization takes place. The resulting oligomers are hydrogenated. The olefins from the isomerization/disproportionation process which are less than 14 carbon atoms or greater than 20 carbon atoms are once again subjected to the isomerization/disproportionation process as above, the 14 to 20 carbon atom cut is oligomerized, and the process is repeated. The resulting hydrogenated oligomerized olefins are useful in internal combustion engines as crankcase lubricants.

Thermal cis-trans isomerization of cistransoidal polyphenylacetylene

Abstract: Solution and solid-state thermal cis-trans isomerization of cis–transoidal polyphenylacetylene was investigated. At temperatures higher than 120°C, cis-trans thermal isomerization in solution is accompanied by cyclization, aromatization, and scission of the polymer chain. Both spectral and kinetics data showed that at temperatures lower than 120°C, not only cis-trans thermal isomerization takes place but also intramolecular cyclization.

Correlation effects in the isomeric cyanides: HNC↔HCN, LiNC↔gLiCN, and BNC↔gBCN

Abstract: Correlated values of the isomerization energy and barrier for the HNC→HCN reaction are obtained from many‐body perturbation theory, including the effects of quadruple excitations. Extended basis sets of better‐than‐triple‐zeta plus double‐polarization quality are used, as well as basis sets including counterpoise and bond‐centered orbitals. The best of these basis sets is sufficient to account for 84% of the valence correlation energy of HCN. These studies predict an isomerization energy for the HNC→HCN rearrangement to be −15±2 kcal/mole, in disagreement with a recent experimental value of −10.3±1. kcal/mole. Correlated isomerization energies of LiCN→LiNC and BCN→BNC are obtained in bases of double‐zeta plus polarization quality. In all cases, correlation stabilizes the cyanide isomer more than the isocyanide. Trends in the series R–NC⇄RCN for R=H, Li, and B are discussed.

Structural intermediates trapped during the folding of ribonuclease A by amide proton exchange.

Abstract: In the folding reaction of the slow-folding species (Us) of ribonuclease A (RNase A), the slow isomerization of wrong proline isomers provides a suitable trap for kinetic folding intermediates at low temperatures (0-10 "C). Partly folded intermediates are known to accumulate before proline isomerization takes place, after which native RNase A is formed. We have been able to measure the protection from amide proton exchange which is provided by structure in the intermediates at different times along the folding pathway. Previous work has shown that, by labeling the amide protons of the unfolded protein before initiating refolding, an early Eevious work has shown that proline isomerization can be used as a kinetic trap for intermediates in the folding of RNase A.' The results can be summarized as follows. (1) There are two classes of unfolded RNase A: a fast-folding class, UF, and a major (80%) slow-folding class, Us (Garel & Baldwin, 1973, 1975a,b; Brandts et al., 1975; Garel et al., 1976; Hag- erman & Baldwin, 1976). (2) The UF + Us interconversion reaction in unfolded RNase A has been shown to be acid catalyzed and to have other specific properties of proline isomerization or of peptide bond isomerization (Schmid & Baldwin, 1978, 1979a), and it is probable that the slow-folding molecules of RNase A have one or more wrong proline iso- mers, as first proposed by Brandts et al. (1975). (3) There is recent evidence that, in the folding of RNase A at low temperatures, structural intermediates accumulate before

Z/e photoisomerization of urocanic acid*

Abstract: — The E ⇆ Z photoisomerization of the title compound (UA) (a naturally occurring sunscreen) has been studied in aqueous solution. At a UA concentration of 6mM and using 313nm excitation, φEz= 0.52, φZE= 0.47 and the photostationary state is 34% E. Under these conditions, loss of UA is minimal. Low energy triplet quenchers fail to impede the isomerization, but the reaction can be induced by several triplet sensitizers. The ET for UA is estimated to be approximately 55 kcal/mol.

Structural reorganizations in lipid bilayer systems: effect of hydration and sterol addition on Raman spectra of dipalmitoylphosphatidylcholine multilayers.

Abstract: Vibrational Raman spectroscopy was used in investigate the conformational behavior of dipalmitoylphosphatidylcholine (DPPC) bilayers perturbed by cholesterol and water, two membrane components whose lipid interactions involve different regions of the bilayer matrix. Upon the addition of cholesterol, an intrinsic membrane constituent, to an anhydrous bilayer in concentrations varying from 7 to 30 mol %, modifications in lateral chain interactions were observed by monitoring spectral changes in the methylene C-H stretching and the CH2 deformation regions. The perturbation in the 1460-cm-1 region was not spectroscopically observed until after the addition of 7 mol % of the sterol. Although chain-chain interactions are altered, no additional trans/gauche isomerization is developed along the hydrocarbon chains. Water, a peripheral bilayer component, was added to the multilayer assembly in the hydration range of 0.3 to 4 molecules of water per lipid molecule. Vibrational spectra characteristic of motions in the head-group, interfacial, and acyl chain regions of the lipid bilayer were observed. These data indicate that hydration confers a mobility to the head-group, glycerol, and carbonyl moieties. Shifts in the CN symmetric and PO2-antisymmetric stretching modes, occurring on the addition of approximately four molecules of water, indicate a conformational rearrangement within the polar head group. After approximately four molecules of water are added to the DPPC system, the spectral features of the gel system [70% (w/w) water] indicate that not further head-group changes nor increases in either acyl chain trans/gauche or lattice disorder arise on further hydration.

A possible role for triplet H2CN+ isomers in the formation of HCN and HNC in interstellar clouds

Abstract: The structures and energies of the lowest triplet states of four isomers of H2CN+ have been determined by self‐consistent field and configuration interaction calculations. When both hydrogen atoms are attached to the nitrogen atom, H2NC+, the molecule has its lowest triplet state energy, which is 97.2 kcal mol−1 above the energy of the linear singlet ground state. The structure has C2v symmetry, with an HCH bond angle of 116.8°, and bond lengths of 1.009 A (H–N) and 1.268 A (N–C). Other isomers investigated include the H2CN+ isomer at 104.7, the cis‐HCNH+ isomer at 105.3, and the trans‐HCNH+ isomer at 113.6 kcal mol−1. The H2CN+ isomer has an unusual ’’carbonium nitrene’’ structure, with a C–N bond length of 1.398 A. It is suggested that the triplet H2NC+ isomer may play a role in determining the relative yields of HCN and HNC from the reaction of C+ and NH3. Specifically, a triplet path is postulated in which C+ and NH3 yield the triplet H2NC+ isomer, which then yields the singlet H2NC+ isomer by phospho...

Theoretical characterization of the potential energy surface of the ground state of the HCO system

Abstract: Large scale, ab initio configuration interaction calculations are reported on the ground state potential energy surface of the HCO system. The calculated equilibrium geometry of HCO of RCH=1.12 A, RCO=1.19 A, and ϑHCO=126° is in good agreement with that determined experimentally, although the calculated H–CO bond strength De=16 kcal/mole is too small by ∼3 kcal/mole. A barrier of 7.5 kcal/mole is found for recombination of H+CO; the height of the barrier is estimated to be too large by 5–6 kcal/mole. The COH isomer, with a calculated equilibrium geometry of ROH=0.98 A, RCO=1.29 A, and ϑCOH=114°, is predicted to lie 40 kcal/mole above the HCO isomer, or 24 kcal/mole above the H+CO limit. For COH the barrier for dissociation of 16 kcal/mole is substantially less than that for isomerization (29 kcal/mole) in spite of the fact that isomerization is energetically more favorable. The COH isomer has not yet been identified experimentally. For the HCO system there seems to be no significant difference between the...

trans/cis Isomerization of Astaxanthin Diacetate/Isolation by HPLC. and Identification by 1H‐NMR. Spectroscopy of Three Mono‐cis‐ and Six Di‐cis‐Isomers

Abstract: Thermal and iodine-catalyzed photochemical trans/cis isomerization of synthetic, racemic astaxanthin diacetate (3,3′-dihydroxy-β,β-carotene-4,4′-dione diacetate) yielded multi-component mixtures of cis-isomers. Separation and isolation of 10 different cis-isomers in quantities between 10 and 70 μg was achieved by HPLC. Investigation of their 270-MHz-FT-1H-NMR. spectra led to the identification of 9 of these isomers, namely the 9-, 13-, and 15-mono-cis-, the 9,9′-, 9,13-, 9,13′-, 9,15-, 13,13′-, and 13,15-di-cis-astaxanthin diacetate.